Method of and apparatus for cooking pulp



Filed NOV. 8, 1954 F W ADAMS EI'AL METHOD OF AND APPARATUS FOR COOKING PULP I I I April 28, 1936.

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Patented A r. 28, 19 36 PATENT oFFicE METHOD OF. AND APPARATUS FOR COOKING PULP Frederick W. Adams, Stoneham, Mass, and George S. Witham, Jr., Lincoln, N. H.

Application November 8,1934, Serial No. 752,058

7 Claims.

This invention relates to methods of cooking sulphite pulp and to the apparatus used in practicing such methods. It aims to efiect economies in such methods, to afford a better control of them, and to improve the quality and yield of the product obtained from them.

,The nature of the invention will be readily understood from the following description when read in connection with the accompanying drawing, and the novel features will be particularly pointed out in-the appended claims.

In the drawing,

Figure 1 is a diagrammatic view of a pulp cooking system or. apparatus embodying this invention; and

Fig. 2 is a plan viewof parts of the apparatus illustrated in Fig. 1.

Preliminary to a detailed description of the system shown in'the drawing, it may be pointed out that a typical method of cooking pulp by the sulphite process may be considered to be' divided roughly into four steps: first, the heating of the charge; second, the penetration of the acid into the chips; third, cooking, and finally, recovery. In a digester having a capacity of, say, ten tons, the heating period may last, in a typical cook, from two to four hours, during which time,

steam is supplied to the charge at a very rapid rate, the digester being vented in the usual man.- ner. By the end of this period the pressure in the digester may have risen to in the neighborhood of seventy-five or eighty pounds per square inch, and thereafter the rate of steam inflow is so reduced as to maintain a substantially constant pressure in the digester. Toward-the end of the heating stage the temperature in the digester has risen to such a point that the acid penetrates the wood and the cooking starts. Due to the nature of the charge in the digester, the circulation of the acid within it proceeds very slowly, the temperatures vary widely in difierent parts of it, and these temperature variations necessarily produce corresponding variations in the cooking conditions. Consequently, the wood in some parts of the digester is penetrated by the acid much earlier in the cooking process than that in other parts, and the chips, therefore, become overcooked in some portions of the digester before those in other parts of it have become cooked to the desired degree. The present invention is especially concerned with these conditions and provides a practical method for correcting them.

The arrangement shown in the drawing comprises a digester 2 which may be of any common or suitable type. It may be equipped with the .usual steam supply connections, suitable apparatus for controlling the flow of'steam in the desired manner, and any other desired or usual auxiliary equipment.

The acid supply system includes a novel ar- 5 rangement comprising two storage tanks indicated, respectively, at 4 and 6. The latter of these tanks is connected through thepipe 8 with the acid system from which it receives its supply of raw acid. It is also connected through the pipe 10;

line I!) and valve i2 with the relief pipe leading from the top of the digester, or with any convenient source of sulphur dioxide suitable for enriching the concentration of the raw acid in the tank 6. The acid from this tank flows through an overflow pipe l4 into the storage tank 4, and this acid may be drawn'off'thr'ough the pipeline IE to the pump l8, and thence forced through the acid supply line into the digester 2. Usually the valve 22 in the pipe line I6 is closed 20 so that acid can flow from the tank 6 into the tank 4 only through the overflow pipe l4. Both these tanks are vented through the pipe connections 21 and 26 to the raw acid system so that any unabsorbed gases in the tanks will not build up a pressure in them, and also for the purpose of preventing the formation'of a vacuum in either tank due to the withdrawal of acid from it, but the tanks are sealed to the surrounding atmosphere through a water trap 28. The acid in both tanks 30 is, therefore, maintained under substantially atmospheric pressure.

According to the present invention the acid stored in the tank 4 is heated to an elevated temperature approximating that used in the cooking ,of the pulp, and this heating operation is performed by conducting low pressure steam into the acid through the pipe connections 30 and the injector 32. This injector is located in the bottom of the tank adjacent to one wall thereof, and the 40 injector arrangement serves both to heat the acid and also to create a continuouscirculation of the acid. -An important problem in preheating acid for use in a pulp digester is to avoid the undue loss of sulphur dioxide gas. We have found, however, that by using the injector arrangement shown, and delivering steam at low pressure, say, for example, twenty pounds, the rapid circulation of acid created by such delivery, and the consequent-immediate mixing of the steam-with 5 the acid and the rapid dispersion'of heat so produced avoids any substantial separation of gas, while at the same time permitting us to heat this body ofacid up to relatively high temperatures,

say, for example, "or F. Usually these 55 tanks are made of wood and consequently, it is not desirable to heat the acid much above the latter figure because of the destructive action of the acid on the walls of the tank itself. If, however, the tank is made of other materials which will stand a higher temperature, then the temperature to which the acid is heated can be raised further. 4

After the digester has been charged with chips, the hot acid from the tank 4 is forced into the digester until it is full. The admission of steam to the digester is immediately begun, as in the ordinary cooking process, but the fact that the entire volume of acid required for the cooking operation is already at an elevated temperature, reduces very substantially the length of the heating period. In a typical case this heating period occupies from two to four hours, during which time the temperature of the digested charge is raised to in the neighborhood of, say,- 200 or 225' F. In addition to shortening the length of time required to complete the cook, we are also enabled by this invention to maintain more uniform conditions throughout the digester. As above stated, the circulation in the digester is very sluggish, and if acid is initially pumped into the digester at normal temperatures, the temperature variations at different points in the digester will initially run all the way from 70 up to approximately the temperature of the steam. On the other hand, if the acid is pumped into the tank at 120 F., then the lower limit of these temperature variations is 120". Because of these more uniform temperature conditions throughout the digester, a more uniform penetration of the acid into the chips in all parts of the digester is produced, a better control of the cooking operation is provided, and both a higher yield and a better quality of pulp are obtained.

Another advantage of this process is that it permits the maintenance of a better heat balance in the steam generating plant since in almost any pulp mill the heating of the acid in the tank 4, or some corresponding container, can proceed continuously, This tank thus acts as a heat accumulator, and by storing heat at times when the demand for steam by the digesters is not heavy, and delivering this heat to the digesters during a period when such demand is great, the extreme variations in load can be reduced. It will be understood, of course, that no attempt has been niade in the drawings to illustrate the relative proportion of the tanks and the digester, but that the capacities of the tanks. necessarily will vary with the requirements of difierent plants, and that the minimum size of the storage tank 4 would be larger than that required to hold the acid supply for arsingle digester. a

A further gain in efliciency, pulp yield and control of temperature conditions'can be effected by combining with the arrangement above described an apparatus for recirculating thepulp digester liquor as the cook progresses, as disclosed in our earlier application. This circulation may be produced by withdrawing acid from the digester through the pipe 34 and forcing it through the pipe connections 35 and 36 back into the digester again, a steam operated injector 38 included in these connections supplying the necessary power and serving, also, to add heat to the acid while it is outside the digester. Instead of using the injector, the acid may be led into the rotary pump 40 driven by the electric motor 42 and forced by it around through the pipe connections, and; if desired, through some kind of a heater 44 such,

for example, as an economizer in the boiler, and returned again to the digester.

During the latter stages of the cook, when the digester must be vented, the liquor and gases forced out through the vent may be returned to the raw gas or recovery systems in the usual manner. Usually, however, some of this acid and relief gas is conducted through the pipe line III to the storage tank 6 where their heat content is conserved and the sulphur dioxide is used in increasing the concentration of the raw acid delivered to this tank from other sources. Preferably a cooler 45 is included in the pipe line H] to, reduce the temperature of the relief gas and acid to suitable values for delivery to the tank 6. The degree of cooling will vary with the conditions at the plant. Usually it is made such that the temperature of the acid in the tank 6 is kept at about F.

While we have herein shown and described a typical embodiment of our invention, it will be understood that the invention may be embodied in other forms without-departing from the spirit or scope thereof. In fact, the details of construction of any individual installation and the temperatures and pressures used necessarily will be determined by the conditions at the plant where such installation is made.

Having thus described our invention, what we desire to claim as new is:

1. That improvement in pulp cooking methods, which consists in storinga supply of acid to be used in the pulp cooking operation, preheating said stored acid to an elevated temperature, maintaininga continuous circulation of the acid substantially throughout said preheating operation, forcing the hot acid into the digester, proceeding with the cooking operation, recirculating the digester liquor during the cooking process, and adding heat to said liquor outside the digester during its recirculation.

2. In a pulp cooking apparatus, the combination of a pulp digester, a tank for holding a quantity .of acid to be used in the digesting process, a steam discharge nozzle in said tank, connections for conducting steam to said nozzle, the nozzle being positioned to discharge the steam into said acid in such a direction as to produce a circulation oi the acid in said tank, a second tank, connections for conducting acid from the I latter to the first tank, connections for conducting raw acid from the raw acid system and relief gas from said digester into said second tank, an-

acid supply line leading from said tank to said digester, and means for forcing the acid through said line into the digester.

3. In a pulp cooking apparatus, the combina tion of a pulp digester, a tank forholding a quantity of acid to be used in the digesting process, a

steam discharge nozzle in said tank, connections for conducting steam to said nozzle, the nozzle being positioned to discharge the steam into said acid in such a direction as to produce a circulation of the acid in said tank, means for sealing said tank from the outside atmosphere but maintaining the acid therein at approximately atmospheric pressure, an acid supply line leading from said tank to said digester, and means for forcing the acid through said line into the digester.

4; In a pulp cooking apparatus, the combination of a pulp digester, a tank for holding a quantity of acid to beused in the digesting process, a steam injector in said tank, connections for conducting steam to said injector, the injector being positioned to discharge the steam into said acid in such a direction as to produce a circulation of the acid in said tank, an acid supply line leading from said tank to said digester, and means for forcing the acid through said line into the digester.

5. That improvement in pulp cooking methods, which consists in storing a supply of acid to be used in the pulp cooking operation, discharging steam into said stored acid and thereby preheating it to. an elevated temperature, causing the discharge of said steam into the acid to maintain a continuous circulation of the acid substantially throughout said preheating operation, forcing the hot acid so preheated into the digester, and proceeding with the cooking operation.

6. That improvement in pulp cooking methods, which consists in storing asupply of acid to be used in the pulp cooking operation, conducting steam into said stored acid to preheat it to an elevated temperature, maintaining a continuous circulation of the acid substantially throughout said preheating operation, forcing the hot acid into the digester, proceeding with the. cooking operation, and maintaining said supply or acid at approximately atmospheric pressure during said preheating operation but sealed from-communication with the atmosphere.

7. In a pulp cooking apparatus, the combination of a pulp .digester, a tank for holding a quantity of acid to be used in the digesting process, a steam discharge nozzle in said-tank, connections for conducting steam from a steam generating plant to said nozzle, the nozzle being positioned to discharge the steam into said acid in such a direction as to produce a circulation of the acid in said tank, an acid supply line leading from said tank to said digester, and means for forcing the acid through said line into the digester.

FREDERICK W. ADAMS. GEORGE S. WITHAM, JR. 

